JP3713412B2

JP3713412B2 - Intermediate frequency circuit of television tuner

Info

Publication number: JP3713412B2
Application number: JP2000048973A
Authority: JP
Inventors: 正喜山本
Original assignee: Alps Electric Co Ltd
Current assignee: Alps Alpine Co Ltd
Priority date: 2000-02-21
Filing date: 2000-02-21
Publication date: 2005-11-09
Anticipated expiration: 2020-02-21
Also published as: JP2001238143A

Description

【０００１】
【発明の属する技術分野】
本発明はテレビジョンチューナの中間周波回路に関する。
【０００２】
【従来の技術】
従来のテレビジョンチューナの中間周波回路（以下単に中間周波回路という）は図６に示すように、混合器３１と中間周波同調回路３２と中間周波増幅器３３とを有している。混合器３１にはテレビジョン信号又はＦＭ放送信号（ＲＦで示す）が入力されると共に、局部発振信号（Ｌｏで示す）が入力される。
混合器３１は、テレビジョン信号受信時にはテレビジョンの中間周波信号を出力し、ＦＭ放送信号受信時にはテレビジョンの中間周波帯における音声中間周波数に周波数変換されたＦＭ放送信号を出力する。
【０００３】
中間周波同調回路３２はバラクタダイオード３２ａと、バラクタダイオード３２ａのカソードに一端が接続され他端が接地されたコンデンサ３２ｂと、互いに直列に接続された二つのインダクタンス素子３２ｃ、３２ｄとを有し、バラクタダイオード３２ａとコンデンサ３２ｂとの直列接続回路に対してインダクタンス素子３２ｃ、３２ｄの直列接続回路が並列に接続され、並列同調回路を構成する。
インダクタンス素子３２ｃと３２ｄとの接続点には電圧Ｖｂが印加され、この電圧はインダクタンス素子３２ｃを介してバラクタダイオード３２ａのカソードに供給される。
【０００４】
さらに、バラクタダイオード３２ａのアノードは、直列接続された二つの抵抗３４、３５を介して接地される。抵抗３４と３５との接続点には切替手段３６によって電圧Ｖｂが印加されるか又は０ボルトの電圧が印加される。
そして、バラクタダイオード３２ａのカソードとインダクタンス素子３２ｃとの接続点が混合器３１の出力端に接続されると共に、結合コンデンサ３７を介して中間周波増幅器３３の入力端に接続される。
【０００５】
以上の構成において、テレビジョン信号受信時には切替手段３６によって抵抗３４と３５との接続点に０ボルトの電圧が印加される。すると、バラクタダイオード３２ａのアノードとカソードとの間には逆バイアス電圧が印加される。このときのバラクタダイオード３２ａの容量値とコンデンサ３２ｂの容量値、及びインダクタンス素子３２ｃ、３２ｄのインダクタンス値によって、中間周波同調回路３２の同調周波数が決定されるが、その周波数は中間周波帯の映像中間周波数Ｐと音声中間周波数Ｓとの間（好ましくは映像中間周波数Ｐと色副搬送波周波数Ｃとの間）に設定され、同調特性は図７のＡに示すようになる。
【０００６】
一方、ＦＭ放送信号受信時には切替手段３６によって抵抗３４と３５との接続点に電圧Ｖｂが印加される。すると、バラクタダイオード３２ａのアノードとカソードとの間はほぼ同電位となり、バラクタダイオード３２ａの容量値は極めて大きくなり、同調周波数は低くなる。このときの同調周波数は中間周波帯の音声中間周波数Ｓとなるように設定され、同調特性は図７のＢに示すようになる。
【０００７】
【発明が解決しようとする課題】
しかしながら、ＦＭ放送信号受信では中間周波同調回路３２の同調容量（バラクタダイオード３２ａとコンデンサ３２ｂとの直列容量）値が大きくなるので、混合器３１の出力インピーダンスと中間周波増幅器３３の入力インピーダンスとを含めた全体の同調回路の負荷Ｑがテレビジョン信号受信時に比較して大きくなり、損失も増加する。そのため、テレビジョン信号受信における同調特性のピーク点よりもＦＭ放送信号受信時における同調特性のピーク点が３ｄＢ程度低くなり、テレビジョンチューナ全体の利得不足を来す。この結果、ＦＭ放送信号受信時の音声感度が低下するという問題が発生する。
【０００８】
また、中間周波増幅器３３の入力インピーダンスが低いので、全体のＱ自体を高くすることが出来ず、同調特性が緩やかとなり、放送周波数が隣接するＦＭ放送信号との混信を起こすという問題があった。
【０００９】
そこで、本発明のテレビジョンチューナの中間周波回路は、中間周波増幅器の入力インピーダンスが低い場合であっても、ＦＭ放送受信時における同調特性のＱを高くすると共に、損失を少なくすることを目的とする。
【００１０】
【課題を解決するための手段】
上記の課題を解決するため、本発明では、テレビジョン信号受信時にはテレビジョンの中間周波信号を出力すると共に、ＦＭ放送信号受信時にはテレビジョンの中間周波帯における音声中間周波数に周波数変換されたＦＭ放送信号を出力する混合器と、前記混合器に接続され、前記テレビジョン信号受信時には前記中間周波帯における映像中間周波数と音声中間周波数との間に同調し、前記ＦＭ放送信号受信時には前記中間周波帯における音声中間周波数に同調する中間周波同調回路と、結合容量手段を介して前記中間周波同調回路に結合された中間周波増幅器とを備え、前記結合容量手段の容量値を大または小の二通りに切り替えるように構成し、前記テレビジョン信号受信時には前記結合容量手段の容量値を大きくし、前記ＦＭ放送信号受信時には小さくした。
【００１１】
また、前記中間周波同調回路を並列同調回路で構成すると共に前記並列同調回路には前記同調周波数を変える第一のバラクタダイオードを設け、前記結合容量手段は互いに直列接続されたコンデンサと第二のバラクタダイオードとで構成し、前記テレビジョン信号受信時には前記第一のバラクタダイオードに第一の逆バイアス電圧を印加すると共に、前記第二のバラクタダイオードを導通状態とし、前記ＦＭ放送信号受信時には前記第一のバラクタダイオードに前記第一の逆バイアス電圧よりも低い第二の逆バイアス電圧を印加すると共に、前記第二のバラクタダイオードに第三の逆バイアス電圧を印加した。
【００１２】
また、前記第一のバラクタダイオードのアノードと前記第二のバラクタダイオードのカソードとを直流的に接続し、前記第一のバラクタダイオードのカソードに第一の電圧を印加すると共に、前記第二のバラクタダイオードのアノードに前記第一の電圧よりも低い第二の電圧を印加し、前記テレビジョン信号受信時には前記第一のバラクタダイオードのアノードと前記第二のバラクタダイオードのカソードとの直流的な接続点の電圧を前記第二の電圧以下とし、前記ＦＭ放送信号受信時には前記直流的な接続点の電圧を前記第一の電圧と前記第二の電圧との間の電圧とした。
【００１３】
なた、互いに直列に接続された二つの抵抗の接続点から分圧電圧を出力する分圧回路を設け、前記二つの抵抗の接続点を前記直流的な接続点に接続し、ＦＭ放送受信時には前記分圧回路に前記第二の電圧よりも大きな電圧を印加すると共に、前記分圧電圧を前記第一の電圧と前記第二の電圧との間の電圧となるように設定した。
【００１４】
また、ＦＭ放送受信時にオンする切替手段を設け、ＦＭ放送受信時に前記第二の電圧を前記切替手段を介して前記分圧回路に印加した。
【００１５】
また、前記混合器は二つの出力端を有する平衡型混合器で構成され、前記中間周波増幅器は二つの入力端を有する平衡型中間周波増幅器で構成され、前記結合容量手段を二つ設け、前記中間周波同調回路の両端を前記平衡型混合器の二つの出力端間に接続し、前記平衡型中間周波増幅器の一方の入力端を前記結合容量手段の一方を介して前記中間周波同調回路の一端に接続し、前記平衡型中間周波増幅器の他方の入力端を前記結合容量手段の他方を介して前記中間周波同調回路の他端に接続した。
【００１６】
また、前記平衡型中間周波増幅器は供給される電源電圧によって前記二つの入力端に直流的なバイアス電圧を発生し、前記第一の電圧を前記電源電圧として前記平衡型中間周波増幅器に供給し、前記結合容量手段における第二のバラクタダイオードの各アノードを前記平衡型中間周波増幅器の二つの入力端にそれぞれ接続した。
【００１７】
【発明の実施の形態】
本発明のテレビジョンチューナの中間周波回路を図１乃至図５を用いて説明する。図１は本発明のテレビジョンチューナの中間周波回路の基本構成を示す回路図、図２はその等価回路図、図３は同調特性図、図４は本発明のテレビジョンチューナの中間周波回路を平衡型で構成した場合の回路図、図５はその等価回路図である。
【００１８】
図１において、混合器１にはテレビジョン信号又はＦＭ放送信号（ＲＦで示す）が入力されると共に、局部発振信号（Ｌｏで示す）が入力される。そして、混合器１は、テレビジョン信号受信時にはテレビジョンの中間周波信号を出力し、ＦＭ放送信号受信時にはテレビジョンの中間周波帯における音声中間周波数に周波数変換されたＦＭ放送信号を出力する。
【００１９】
中間周波同調回路２は第一のバラクタダイオード２ａと、第一のバラクタダイオード２ａのカソードに一端が接続され他端が接地されたコンデンサ２ｂと、互いに直列に接続された二つのインダクタンス素子２ｃ、２ｄとを有し、第一のバラクタダイオード２ａとコンデンサ２ｂとの直列接続回路に対してインダクタンス素子２ｃ、２ｄの直列接続回路が並列に接続され、並列同調回路を構成する。
インダクタンス素子２ｃと２ｄとの接続点には第一の電圧Ｖｂ１が印加され、この電圧はインダクタンス素子２ｃを介して第一のバラクタダイオード２ａのカソードに供給される。
【００２０】
そして、第一のバラクタダイオード２ａのカソードとインダクタンス素子２ｃとの接続点が混合器１の出力端に接続されると共に、結合容量手段３に接続される。結合容量手段３は直流カットコンデンサ４を介して中間周波増幅器５の入力端に接続される。
結合容量手段３は互いに直列に接続された結合コンデンサ３ａと第二のバラクタダイオード３ｂとから構成され、第二のバラクタダイオード３ｂのアノードが直流カットコンデンサ４に接続される。直流カットコンデンサ４のインピーダンスは無視し得る程度になっている。
【００２１】
そして、第一のバラクタダイオード２ａのアノードと第二のバラクタダイオード３ｂとが高周波阻止用の二つの直列接続抵抗６，７を介して直流的に接続される。又、第二のバラクタダイオード３ｂのアノードは直列接続された抵抗８、９の接続点に接続される。この抵抗８、９は第一の電圧を分圧するものであり、一方の抵抗８に第一の電圧が印加され、他方の抵抗９は接地される。そして、抵抗８と９との接続点に現れる分圧電圧が第二の電圧Ｖｂ２となる。
【００２２】
直列接続された二つの抵抗６、７の接続点は分圧回路１０を構成する二つの抵抗１０ａ、１０ｂの接続点に接続される。抵抗１０ｂの一点は接地され、抵抗１０ａの一端は切替手段手段１１に接続される。切替手段１１は少なくとも二つの端子１１ａ、１１ｂを有し、この二端子１１ａ、１１ｂ間がオン（導通）又はオフ（非導通）に切り替えられ、抵抗１０ｂの一端は端子１０ａに接続され、端子１１ｂには第一の電圧が印加される。図示ではもう一つの端子１１ｃを有し、端子１１ａ、１１ｂ間がオフのときには端子１１ａと端子１１ｃとがオンとなるように動作する。端子１１ｃは抵抗１２を介して接地される。端子１１ｃと抵抗１２とは必ずしも必要ではない。
そして、端子１１ａ、１１ｂ間がオンとなったときの分圧回路１０における分圧電圧（即ち、抵抗１０ａと１０ｂとの接続点の電圧）は抵抗８と９との接続点における第二の電圧よりも高くなるように各抵抗８、９、１０ａ、１０ｂの値が設定されている。
【００２３】
以上の構成において、混合器１の出力インピーダンスをＺｏ、中間周波増幅器５の入力インピーダンスをＺｉとすれば、全体の等価回路は図２に示すようになる。そして、テレビジョン信号受信時には切替手段１１の端子１１ａと１１ｂとの間をオフする。すると、第二のバラクタダイオード３ｂは順方向に電流が流れてオンとなり、第一のバラクタダイオード２ａには第一の逆バイアス電圧が印加される。その結果、結合容量手段３の容量値は結合コンデンサ３ａの容量値のみとなって容量値が大きくなると共に、中間周波同調回路２の同調周波数は、例えば、テレビジョンの中間周波帯における映像中間周波数と音声中間周波数との間（好ましくは映像中間周波数Ｐと色副搬送波周波数Ｃとの間）となるように設定される。この状態の同調特性は図３のＡに示す。同調特性は、例えば中間周波増幅器５の入力端で観測される。
【００２４】
一方、ＦＭ放送信号受信時には切替手段１１の端子１１ａと１１ｂとの間をオンする。すると、分圧回路１０の分圧電圧は第一の電圧よりも低く、第二の電圧よりも高くなる。そして、第一のバラクタダイオード２ａには第二の逆バイアス電圧が印加され、第二のバラクタダイオード３ｂには第三の逆バイアス電圧が印加される。第二の逆バイアス電圧は第一の逆バイアス電圧よりも低い。
【００２５】
その結果、第一のバラクタダイオードの容量値が大きくなって中間周波同調回路２の同調周波数は低くなるが、その周波数はテレビジョンの中間周波数帯における音声中間周波数に一致するようになっている。
一方、第二のバラクタダイオード３ｂにも逆バイアス電圧が印加されることで、結合容量手段３全体の容量値は結合コンデンサ３ａ単独の容量値よりも小さくなる。
【００２６】
結合容量手段３の容量値が小さくなることは中間周波同調回路２に接続される負荷としての中間周波増幅器５の入力インピーダンスが見かけ上大きくなったことになる。そのため、混合器１の出力インピーダンスと中間周波増幅器５の入力インピーダンスとを含めた全体の負荷Ｑが高くなると共に損失も少なくなり、その同調特性は図３のＢに示すように急峻になると共に、ピーク点が高くなる。実際にはテレビジョン信号受信時における同調特性のピーク点よりも４ｄＢ程度高くなる。
【００２７】
本発明のテレビジョンチューナの中間周波回路は、図４に示すように平衡型の回路で構成することも出来る。図４において、混合器２１と中間周波増幅器２２はそれぞれが平衡型回路で構成されている。中間周波増幅器２２には第一の電圧が電源電圧として印加される。そして、中間周波同調回路２の両端は混合器２１の二つの出力端の間に接続される。また、中間周波同調回路２の両端はそれぞれ結合容量手段３、３によって中間周波増幅器２２の二つの入力端に接続されるが、第二のバラクタダイオード３ｂ、３ｂの各アノードが直接中間周波増幅器２２の二つの入力端に接続される。そして第二のバラクタダイオード３ｂ、３ｂのカソードがそれぞれ高周波阻止用の抵抗６、６によって抵抗７に接続される。以上の構成の等価回路図は図５に示される。
【００２８】
なお、中間周波増幅器２２は、印加される電源電圧によって、入力端に増幅素子用のバイアス電圧が与えられるものであるので、このバイアス電圧が第二の電圧として第二のバラクタダイオード３ｂ、３ｂのアノードに印加される。
以上説明した平衡型のテレビジョンチューナの中間周波回路の動作は図１の構成の動作と同じであるのでその説明は省略する。
【００２９】
【発明の効果】
以上のように、混合器と、混合器に接続されてテレビジョン信号受信時には映像中間周波数と音声中間周波数との間に同調しＦＭ放送信号受信時には音声中間周波数に同調する中間周波同調回路と、結合容量手段を介して中間周波同調回路に結合された中間周波増幅器とを備え、結合容量手段の容量値を大または小の二通りに切り替えるように構成し、テレビジョン信号受信時には結合容量手段の容量値を大きくし、ＦＭ放送信号受信時には小さくしたので、中間周波同調回路に接続される負荷としての中間周波増幅器の入力インピーダンスが見かけ上大きくなり、そのため、混合器の出力インピーダンスと中間周波増幅器の入力インピーダンスとを含めた全体の負荷Ｑが高くなると共に損失も少なくなる。そして、同調特性は急峻になると共に、ピーク点が高くなる。この結果、ＦＭ放送信号受信時には隣接するＦＭ放送信号との混信を起こすこともなく、又受信感度も向上する。
【００３０】
また、中間周波同調回路を並列同調回路で構成し、並列同調回路に第一のバラクタダイオードを設け、結合容量手段は互いに直列接続されたコンデンサと第二のバラクタダイオードとで構成し、テレビジョン信号受信時には第一のバラクタダイオードに第一の逆バイアス電圧を印加すると共に、第二のバラクタダイオードを導通状態とし、ＦＭ放送信号受信時には第一のバラクタダイオードに第一の逆バイアス電圧よりも低い第二の逆バイアス電圧を印加すると共に、第二のバラクタダイオードに第三の逆バイアス電圧を印加したので、受信信号が変わる場合の同調周波数と結合容量手段の容量値と切替は簡単になる。
【００３１】
また、第一のバラクタダイオードのアノードと第二のバラクタダイオードのカソードとを直流的に接続し、第一のバラクタダイオードのカソードに第一の電圧を印加すると共に、第二のバラクタダイオードのアノードに第一の電圧よりも低い第二の電圧を印加し、テレビジョン信号受信時には第一のバラクタダイオードのアノードと第二のバラクタダイオードのカソードとの直流的な接続点の電圧を第二の電圧以下とし、ＦＭ放送信号受信時には直流的な接続点の電圧を第一の電圧と第二の電圧との間の電圧としたので、第一のバラクタダイオードと第二のバラクタダイオードとに印加する逆バイアス電圧に切替が連動できる。よって構成が簡単である。
【００３２】
なた、互いに直列に接続された二つの抵抗の接続点から分圧電圧を出力する分圧回路を設け、二つの抵抗の接続点を第一のバラクタダイオードのアノードと第二のバラクタダイオードのカソードと間の直流的な接続点に接続し、ＦＭ放送受信時には分圧回路に第二の電圧よりも大きな電圧を印加すると共に、分圧電圧を第一の電圧と第二の電圧との間の電圧となるように設定したので、分圧回路の抵抗の設定によって上記接続点の電圧が切り替えられる。
【００３３】
また、ＦＭ放送受信時にオンする切替手段を設け、ＦＭ放送受信時に第二の電圧を切替手段を介して分圧回路に印加したので、分圧回路に印加する電圧を別に設けなくてもよい。
【００３４】
また、平衡型混合器と平衡型中間周波増幅器を用い、中間周波同調回路の両端を平衡型混合器の二つの出力端間に接続し、平衡型中間周波増幅器の一方の入力端を結合容量手段の一方を介して中間周波同調回路の一端に接続し、平衡型中間周波増幅器の他方の入力端を結合容量手段の他方を介して中間周波同調回路の他端に接続したので、平衡型の中間周波回路を簡単に構成できる。
【００３５】
また、平衡型中間周波増幅器は供給される電源電圧によって二つの入力端に直流的なバイアス電圧を発生し、第一の電圧を前記電源電圧として平衡型中間周波増幅器に供給し、結合容量手段における第二のバラクタダイオードの各アノードを平衡型中間周波増幅器の二つの入力端にそれぞれ接続したので、結合容量手段と中間周波増幅器との間に設けていた直流カットコンデンサを削除できる。
【図面の簡単な説明】
【図１】本発明のテレビジョンチューナの中間周波回路の基本構成を示す回路図である。
【図２】図１に示すテレビジョンチューナの中間周波回路の等価回路図である。
【図３】本発明のテレビジョンチューナの中間周波回路における同調特性図である。
【図４】本発明のテレビジョンチューナの中間周波回路を平衡型で構成した場合の回路図である。
【図５】図４に示すテレビジョンチューナの中間周波回路の等価回路図である。
【図６】従来のテレビジョンチューナの中間周波回路の構成を示す回路図である。
【図７】従来のテレビジョンチューナの中間周波回路における同調特性図である。
【符号の説明】
１、２１混合器
２中間周波同調回路
２ａ第一のバラクタダイオード
２ｂコンデンサ
２ｃ、２ｄインダクタンス素子
３結合容量手段
３ａ結合コンデンサ
３ｂ第二のバラクタダイオード
４直流カットコンデンサ
５、２２中間周波増幅器
６、７高周波阻止用の抵抗
８、９抵抗
１０分圧回路
１０ａ、１０ｂ抵抗
１１切替手段
１１ａ、１１ｂ、１１ｃ端子
１２抵抗[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an intermediate frequency circuit of a television tuner.
[0002]
[Prior art]
A conventional television tuner intermediate frequency circuit (hereinafter simply referred to as an intermediate frequency circuit) includes a mixer 31, an intermediate frequency tuning circuit 32, and an intermediate frequency amplifier 33 as shown in FIG. The mixer 31 receives a television signal or an FM broadcast signal (indicated by RF) and a local oscillation signal (indicated by Lo).
The mixer 31 outputs a television intermediate frequency signal when receiving a television signal, and outputs an FM broadcast signal frequency-converted to an audio intermediate frequency in the television intermediate frequency band when receiving an FM broadcast signal.
[0003]
The intermediate frequency tuning circuit 32 includes a varactor diode 32a, a capacitor 32b having one end connected to the cathode of the varactor diode 32a and the other end grounded, and two inductance elements 32c and 32d connected in series with each other. A series connection circuit of inductance elements 32c and 32d is connected in parallel to a series connection circuit of the diode 32a and the capacitor 32b, thereby forming a parallel tuning circuit.
A voltage Vb is applied to the connection point between the inductance elements 32c and 32d, and this voltage is supplied to the cathode of the varactor diode 32a via the inductance element 32c.
[0004]
Further, the anode of the varactor diode 32a is grounded via two resistors 34 and 35 connected in series. A voltage Vb is applied to the connection point between the resistors 34 and 35 by the switching means 36 or a voltage of 0 volts is applied.
A connection point between the cathode of the varactor diode 32 a and the inductance element 32 c is connected to the output terminal of the mixer 31 and is connected to the input terminal of the intermediate frequency amplifier 33 via the coupling capacitor 37.
[0005]
In the above configuration, a voltage of 0 volts is applied to the connection point between the resistors 34 and 35 by the switching means 36 when receiving a television signal. Then, a reverse bias voltage is applied between the anode and cathode of the varactor diode 32a. At this time, the tuning frequency of the intermediate frequency tuning circuit 32 is determined by the capacitance value of the varactor diode 32a, the capacitance value of the capacitor 32b, and the inductance values of the inductance elements 32c and 32d. It is set between the frequency P and the audio intermediate frequency S (preferably between the video intermediate frequency P and the color subcarrier frequency C), and the tuning characteristic is as shown in A of FIG.
[0006]
On the other hand, the voltage Vb is applied to the connection point between the resistors 34 and 35 by the switching means 36 when receiving the FM broadcast signal. Then, the anode and cathode of the varactor diode 32a are almost at the same potential, the capacitance value of the varactor diode 32a becomes extremely large, and the tuning frequency becomes low. The tuning frequency at this time is set to be the audio intermediate frequency S in the intermediate frequency band, and the tuning characteristics are as shown in B of FIG.
[0007]
[Problems to be solved by the invention]
However, in FM broadcast signal reception, the tuning capacitance (series capacitance of the varactor diode 32a and the capacitor 32b) of the intermediate frequency tuning circuit 32 becomes large, so that the output impedance of the mixer 31 and the input impedance of the intermediate frequency amplifier 33 are included. In addition, the load Q of the entire tuning circuit becomes larger than when a television signal is received, and the loss also increases. For this reason, the peak point of the tuning characteristic at the time of FM broadcast signal reception is about 3 dB lower than the peak point of the tuning characteristic at the time of television signal reception, resulting in insufficient gain of the entire television tuner. As a result, there arises a problem that the voice sensitivity at the time of FM broadcast signal reception is lowered.
[0008]
In addition, since the input impedance of the intermediate frequency amplifier 33 is low, the overall Q itself cannot be increased, the tuning characteristic becomes gradual, and there is a problem that the broadcast frequency causes interference with the adjacent FM broadcast signal.
[0009]
Therefore, an object of the intermediate frequency circuit of the television tuner of the present invention is to increase the Q of the tuning characteristic at the time of FM broadcast reception and reduce the loss even when the input impedance of the intermediate frequency amplifier is low. To do.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention outputs an intermediate frequency signal of a television when receiving a television signal, and an FM broadcast frequency-converted to an audio intermediate frequency in the intermediate frequency band of the television when receiving an FM broadcast signal. A mixer that outputs a signal, and is connected to the mixer, and is tuned between a video intermediate frequency and an audio intermediate frequency in the intermediate frequency band when the television signal is received, and the intermediate frequency band when the FM broadcast signal is received. And an intermediate frequency amplifier coupled to the intermediate frequency tuning circuit via a coupling capacitor means, and the capacitance value of the coupling capacitor means is divided into two types, large or small. The FM broadcast signal is configured to increase the capacitance value of the coupling capacitor means when receiving the television signal. It was small at the time of signal.
[0011]
The intermediate frequency tuning circuit is formed of a parallel tuning circuit, and the parallel tuning circuit is provided with a first varactor diode that changes the tuning frequency, and the coupling capacitor means includes a capacitor and a second varactor connected in series with each other. A first reverse bias voltage is applied to the first varactor diode when the television signal is received, and the second varactor diode is turned on, and when receiving the FM broadcast signal, the first varactor diode is turned on. A second reverse bias voltage lower than the first reverse bias voltage was applied to the first varactor diode, and a third reverse bias voltage was applied to the second varactor diode.
[0012]
In addition, the anode of the first varactor diode and the cathode of the second varactor diode are connected in a DC manner, a first voltage is applied to the cathode of the first varactor diode, and the second varactor A second voltage lower than the first voltage is applied to the anode of the diode, and when the television signal is received, a direct current connection point between the anode of the first varactor diode and the cathode of the second varactor diode Is set to be equal to or lower than the second voltage, and when receiving the FM broadcast signal, the voltage at the DC connection point is set to a voltage between the first voltage and the second voltage.
[0013]
In addition, a voltage dividing circuit that outputs a divided voltage from a connection point of two resistors connected in series with each other is provided, and the connection point of the two resistors is connected to the DC connection point, so that FM broadcast is received. A voltage higher than the second voltage was applied to the voltage dividing circuit, and the divided voltage was set to be a voltage between the first voltage and the second voltage.
[0014]
Also, switching means that is turned on when receiving FM broadcast is provided, and the second voltage is applied to the voltage dividing circuit via the switching means when receiving FM broadcast.
[0015]
Further, the mixer is composed of a balanced mixer having two output ends, the intermediate frequency amplifier is composed of a balanced intermediate frequency amplifier having two input ends, and two coupling capacitance means are provided, Both ends of the intermediate frequency tuning circuit are connected between the two output terminals of the balanced mixer, and one input terminal of the balanced intermediate frequency amplifier is connected to one end of the intermediate frequency tuning circuit via one of the coupling capacitor means. And the other input end of the balanced intermediate frequency amplifier is connected to the other end of the intermediate frequency tuning circuit via the other of the coupling capacitor means.
[0016]
The balanced intermediate frequency amplifier generates a DC bias voltage at the two input terminals according to the supplied power supply voltage, and supplies the first voltage as the power supply voltage to the balanced intermediate frequency amplifier. Each anode of the second varactor diode in the coupling capacitor means was connected to two input terminals of the balanced intermediate frequency amplifier.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The intermediate frequency circuit of the television tuner of the present invention will be described with reference to FIGS. 1 is a circuit diagram showing a basic configuration of an intermediate frequency circuit of a television tuner of the present invention, FIG. 2 is an equivalent circuit diagram thereof, FIG. 3 is a tuning characteristic diagram, and FIG. 4 is an intermediate frequency circuit of the television tuner of the present invention. FIG. 5 is an equivalent circuit diagram of a balanced configuration.
[0018]
In FIG. 1, a mixer 1 receives a television signal or FM broadcast signal (indicated by RF) and a local oscillation signal (indicated by Lo). The mixer 1 outputs a television intermediate frequency signal when receiving a television signal, and outputs an FM broadcast signal frequency-converted to an audio intermediate frequency in the television intermediate frequency band when receiving an FM broadcast signal.
[0019]
The intermediate frequency tuning circuit 2 includes a first varactor diode 2a, a capacitor 2b having one end connected to the cathode of the first varactor diode 2a and the other end grounded, and two inductance elements 2c and 2d connected in series with each other. The series connection circuit of the inductance elements 2c and 2d is connected in parallel to the series connection circuit of the first varactor diode 2a and the capacitor 2b to constitute a parallel tuning circuit.
A first voltage Vb1 is applied to a connection point between the inductance elements 2c and 2d, and this voltage is supplied to the cathode of the first varactor diode 2a via the inductance element 2c.
[0020]
A connection point between the cathode of the first varactor diode 2 a and the inductance element 2 c is connected to the output terminal of the mixer 1 and to the coupling capacitor means 3. The coupling capacitor means 3 is connected to the input terminal of the intermediate frequency amplifier 5 through a DC cut capacitor 4.
The coupling capacitor means 3 is composed of a coupling capacitor 3 a and a second varactor diode 3 b connected in series, and the anode of the second varactor diode 3 b is connected to the DC cut capacitor 4. The impedance of the DC cut capacitor 4 is negligible.
[0021]
The anode of the first varactor diode 2a and the second varactor diode 3b are connected in a direct current manner through two series connection resistors 6 and 7 for blocking high frequency. The anode of the second varactor diode 3b is connected to the connection point of the resistors 8 and 9 connected in series. The resistors 8 and 9 divide the first voltage, the first voltage is applied to one resistor 8, and the other resistor 9 is grounded. The divided voltage appearing at the connection point between the resistors 8 and 9 becomes the second voltage Vb2.
[0022]
A connection point between the two resistors 6 and 7 connected in series is connected to a connection point between the two resistors 10 a and 10 b constituting the voltage dividing circuit 10. One point of the resistor 10b is grounded, and one end of the resistor 10a is connected to the switching means 11. The switching means 11 has at least two terminals 11a and 11b, the two terminals 11a and 11b are switched on (conductive) or off (non-conductive), one end of the resistor 10b is connected to the terminal 10a, and the terminal 11b A first voltage is applied to. In the figure, there is another terminal 11c, and when the terminal 11a, 11b is off, the terminal 11a and the terminal 11c are turned on. The terminal 11c is grounded through the resistor 12. The terminal 11c and the resistor 12 are not necessarily required.
The divided voltage in the voltage dividing circuit 10 when the terminals 11a and 11b are turned on (that is, the voltage at the connection point between the resistors 10a and 10b) is the second voltage at the connection point between the resistors 8 and 9. The values of the resistors 8, 9, 10a, and 10b are set so as to be higher than those.
[0023]
In the above configuration, if the output impedance of the mixer 1 is Zo and the input impedance of the intermediate frequency amplifier 5 is Zi, the entire equivalent circuit is as shown in FIG. When receiving a television signal, the terminal 11a and 11b of the switching means 11 are turned off. Then, a current flows in the second varactor diode 3b in the forward direction and turns on, and the first reverse bias voltage is applied to the first varactor diode 2a. As a result, the capacitance value of the coupling capacitor means 3 becomes only the capacitance value of the coupling capacitor 3a, and the capacitance value becomes large. And the audio intermediate frequency (preferably between the video intermediate frequency P and the color subcarrier frequency C). The tuning characteristics in this state are shown in FIG. The tuning characteristic is observed at the input end of the intermediate frequency amplifier 5, for example.
[0024]
On the other hand, when the FM broadcast signal is received, the terminals 11a and 11b of the switching means 11 are turned on. Then, the divided voltage of the voltage dividing circuit 10 is lower than the first voltage and higher than the second voltage. The second reverse bias voltage is applied to the first varactor diode 2a, and the third reverse bias voltage is applied to the second varactor diode 3b. The second reverse bias voltage is lower than the first reverse bias voltage.
[0025]
As a result, the capacitance value of the first varactor diode increases and the tuning frequency of the intermediate frequency tuning circuit 2 decreases, but the frequency matches the audio intermediate frequency in the television intermediate frequency band.
On the other hand, the reverse bias voltage is also applied to the second varactor diode 3b, so that the capacitance value of the entire coupling capacitance means 3 becomes smaller than the capacitance value of the coupling capacitor 3a alone.
[0026]
Decreasing the capacitance value of the coupling capacitor means 3 apparently increases the input impedance of the intermediate frequency amplifier 5 as a load connected to the intermediate frequency tuning circuit 2. Therefore, the overall load Q including the output impedance of the mixer 1 and the input impedance of the intermediate frequency amplifier 5 is increased and the loss is reduced, and the tuning characteristic becomes steep as shown in FIG. The peak point becomes higher. Actually, it is about 4 dB higher than the peak point of the tuning characteristics when receiving a television signal.
[0027]
The intermediate frequency circuit of the television tuner of the present invention can also be constituted by a balanced circuit as shown in FIG. In FIG. 4, each of the mixer 21 and the intermediate frequency amplifier 22 is constituted by a balanced circuit. A first voltage is applied to the intermediate frequency amplifier 22 as a power supply voltage. The both ends of the intermediate frequency tuning circuit 2 are connected between the two output ends of the mixer 21. Further, both ends of the intermediate frequency tuning circuit 2 are connected to the two input ends of the intermediate frequency amplifier 22 by the coupling capacitor means 3 and 3, respectively, but the anodes of the second varactor diodes 3b and 3b are directly connected to the intermediate frequency amplifier 22. Are connected to the two input terminals. The cathodes of the second varactor diodes 3b and 3b are connected to the resistor 7 by resistors 6 and 6 for blocking high frequency, respectively. An equivalent circuit diagram of the above configuration is shown in FIG.
[0028]
Since the intermediate frequency amplifier 22 is provided with a bias voltage for the amplifying element at the input terminal by the applied power supply voltage, this bias voltage is used as the second voltage of the second varactor diodes 3b and 3b. Applied to the anode.
Since the operation of the intermediate frequency circuit of the balanced television tuner described above is the same as the operation of the configuration of FIG. 1, the description thereof is omitted.
[0029]
【The invention's effect】
As described above, a mixer, an intermediate frequency tuning circuit that is connected to the mixer and tunes between the video intermediate frequency and the audio intermediate frequency when receiving a television signal and tunes to the audio intermediate frequency when receiving an FM broadcast signal; An intermediate frequency amplifier coupled to the intermediate frequency tuning circuit via the coupling capacitor means, and configured to switch the capacitance value of the coupling capacitor means between two, large and small, and when receiving a television signal, Since the capacitance value is increased and decreased when receiving FM broadcast signals, the input impedance of the intermediate frequency amplifier as a load connected to the intermediate frequency tuning circuit is apparently increased. Therefore, the output impedance of the mixer and the intermediate frequency amplifier The overall load Q including the input impedance is increased and the loss is reduced. Then, the tuning characteristics become steep and the peak point becomes high. As a result, when receiving FM broadcast signals, there is no interference with adjacent FM broadcast signals, and reception sensitivity is improved.
[0030]
Further, the intermediate frequency tuning circuit is constituted by a parallel tuning circuit, a first varactor diode is provided in the parallel tuning circuit, and the coupling capacitance means is constituted by a capacitor and a second varactor diode connected in series with each other, and the television signal When receiving, a first reverse bias voltage is applied to the first varactor diode, and the second varactor diode is turned on. When receiving an FM broadcast signal, the first varactor diode has a first reverse bias voltage lower than the first reverse bias voltage. Since the second reverse bias voltage is applied and the third reverse bias voltage is applied to the second varactor diode, the switching between the tuning frequency and the capacitance value of the coupling capacitor means when the received signal changes is simplified.
[0031]
Further, the anode of the first varactor diode and the cathode of the second varactor diode are connected in a direct current manner, a first voltage is applied to the cathode of the first varactor diode, and the anode of the second varactor diode is applied. When a second voltage lower than the first voltage is applied and a television signal is received, the voltage at the DC connection point between the anode of the first varactor diode and the cathode of the second varactor diode is less than the second voltage. When the FM broadcast signal is received, the voltage at the DC connection point is set to a voltage between the first voltage and the second voltage, so that the reverse bias applied to the first varactor diode and the second varactor diode is applied. Switching can be linked to the voltage. Therefore, the configuration is simple.
[0032]
In addition, a voltage dividing circuit for outputting a divided voltage from a connection point of two resistors connected in series with each other is provided, and the connection point of the two resistors is set as the anode of the first varactor diode and the cathode of the second varactor diode. Is connected to a DC connection point between them, and when receiving FM broadcast, a voltage higher than the second voltage is applied to the voltage dividing circuit, and the divided voltage is set between the first voltage and the second voltage. Since the voltage is set to be a voltage, the voltage at the connection point is switched by setting the resistance of the voltage dividing circuit.
[0033]
In addition, since the switching unit that is turned on when receiving the FM broadcast is provided and the second voltage is applied to the voltage dividing circuit via the switching unit when the FM broadcast is received, the voltage applied to the voltage dividing circuit may not be provided separately.
[0034]
Also, using a balanced mixer and balanced intermediate frequency amplifier, both ends of the intermediate frequency tuning circuit are connected between the two output terminals of the balanced mixer, and one input terminal of the balanced intermediate frequency amplifier is connected to the coupling capacitor means. Is connected to one end of the intermediate frequency tuning circuit through one of the two, and the other input end of the balanced intermediate frequency amplifier is connected to the other end of the intermediate frequency tuning circuit through the other of the coupling capacitor means. A frequency circuit can be easily configured.
[0035]
The balanced intermediate frequency amplifier generates a DC bias voltage at the two input terminals according to the supplied power supply voltage, and supplies the first voltage as the power supply voltage to the balanced intermediate frequency amplifier. Since each anode of the second varactor diode is connected to the two input terminals of the balanced intermediate frequency amplifier, the DC cut capacitor provided between the coupling capacitor means and the intermediate frequency amplifier can be eliminated.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a basic configuration of an intermediate frequency circuit of a television tuner of the present invention.
FIG. 2 is an equivalent circuit diagram of an intermediate frequency circuit of the television tuner shown in FIG.
FIG. 3 is a tuning characteristic diagram in an intermediate frequency circuit of the television tuner of the present invention.
FIG. 4 is a circuit diagram when the intermediate frequency circuit of the television tuner of the present invention is configured in a balanced type.
5 is an equivalent circuit diagram of an intermediate frequency circuit of the television tuner shown in FIG. 4. FIG.
FIG. 6 is a circuit diagram showing a configuration of an intermediate frequency circuit of a conventional television tuner.
FIG. 7 is a tuning characteristic diagram in an intermediate frequency circuit of a conventional television tuner.
[Explanation of symbols]
1, 21 Mixer 2 Intermediate frequency tuning circuit 2a First varactor diode 2b Capacitor 2c, 2d Inductance element 3 Coupling capacitance means 3a Coupling capacitor 3b Second varactor diode 4 DC cut capacitors 5, 22 Intermediate frequency amplifiers 6, 7 High frequency Resistor for blocking 8, 9 Resistor 10 Voltage dividing circuit 10a, 10b Resistor 11 Switching means 11a, 11b, 11c Terminal 12 Resistor

Claims

A mixer that outputs an intermediate frequency signal of a television when receiving a television signal, and outputs an FM broadcast signal that is frequency-converted to an audio intermediate frequency in the intermediate frequency band of the television when receiving an FM broadcast signal; An intermediate frequency tuning circuit that is tuned between the video intermediate frequency and the audio intermediate frequency in the intermediate frequency band when receiving the television signal, and that is tuned to the audio intermediate frequency in the intermediate frequency band when receiving the FM broadcast signal; An intermediate frequency amplifier coupled to the intermediate frequency tuning circuit via a coupling capacitor means, and configured to switch the capacitance value of the coupling capacitor means between large and small, and at the time of receiving the television signal A capacity value of the coupling capacity means is increased and decreased when receiving the FM broadcast signal. Intermediate frequency circuit of vision tuner.

The intermediate frequency tuning circuit is constituted by a parallel tuning circuit, and the parallel tuning circuit is provided with a first varactor diode that changes the tuning frequency, and the coupling capacitor means includes a capacitor and a second varactor diode connected in series with each other. The first varactor diode is applied to the first varactor diode when the television signal is received, the second varactor diode is turned on, and the first varactor is received when the FM broadcast signal is received. 2. The television according to claim 1, wherein a second reverse bias voltage lower than the first reverse bias voltage is applied to the diode, and a third reverse bias voltage is applied to the second varactor diode. John Tuner's intermediate frequency circuit.

The anode of the first varactor diode and the cathode of the second varactor diode are connected in a direct current manner, a first voltage is applied to the cathode of the first varactor diode, and the second varactor diode A voltage at a DC connection point between the anode of the first varactor diode and the cathode of the second varactor diode is applied when a second voltage lower than the first voltage is applied to the anode and the television signal is received. The voltage at the DC connection point is set to a voltage between the first voltage and the second voltage when the FM broadcast signal is received. An intermediate frequency circuit of the described television tuner.

A voltage dividing circuit for outputting a divided voltage from a connection point of two resistors connected in series is provided, the connection point of the two resistors is connected to the DC connection point, and the divided voltage is received when receiving FM broadcast. The voltage higher than the second voltage is applied to the circuit, and the divided voltage is set to be a voltage between the first voltage and the second voltage. 3. An intermediate frequency circuit of a television tuner according to 3.

5. The television tuner intermediate frequency according to claim 4, further comprising switching means for turning on when receiving FM broadcast, wherein said second voltage is applied to said voltage dividing circuit via said switching means when receiving FM broadcast. circuit.

The mixer is composed of a balanced mixer having two output ends, the intermediate frequency amplifier is composed of a balanced intermediate frequency amplifier having two input ends, two coupling capacitance means are provided, and the intermediate frequency amplifier is provided. Both ends of the tuning circuit are connected between the two output terminals of the balanced mixer, and one input terminal of the balanced intermediate frequency amplifier is connected to one end of the intermediate frequency tuning circuit through one of the coupling capacitor means. 6. The other input terminal of the balanced intermediate frequency amplifier is connected to the other end of the intermediate frequency tuning circuit through the other of the coupling capacitor means. An intermediate frequency circuit for a television tuner.

The balanced intermediate frequency amplifier generates a DC bias voltage at the two input terminals according to the supplied power supply voltage, supplies the first voltage as the power supply voltage to the balanced intermediate frequency amplifier, and 7. The television tuner intermediate frequency circuit according to claim 6, wherein each anode of the second varactor diode in the capacitor means is connected to two input terminals of the balanced intermediate frequency amplifier.